hi i have a gravity fed system with a glow worm ultimate 50 boiler i dont appear to have either a 3 port valve or a header tank in my loft.the problem is that when i turn my heating on it only stays warm for a couple of minutes then goes luke warm at best.i replaced the pump and did a drain down and refill i have bled the pump and all of the radiators(the upstairs one are generally quite warm)the boiler and pipes to the boiler are hot but when i switch the roomstat control on to start the heating it appears to use whatever water is around the boiler and then goes cold also when i turn on the heating the boiler doesnt fire up can someone please help thanks